Interpretive Summary: Rice sheath blight disease caused by the microscopic fungus Rhizoctonia solani is increasing in importance worldwide due to changes in agricultural practices. The dense plantings of rice, while often yielding more rice per acre, also result in more crowded and thus more humid conditions that favor harmful fungi. Although programs are underway to breed rice plants resistant to this disease, little is known about fungal pathogen. Information on the life-style of the pathogen is critical if plant breeders are to be successful. Many fungi harmful to plants can be managed with relative ease since they reproduce by simple budding or other methods that result in offspring that are essentially carbon copies of themselves. On the other hand, other fungi can mate and produce offspring by sexual reproduction. Sexual reproduction leads to offspring that can have many different combinations of traits originating from the parents. Sexually reproducing pathogens therefore can be a more difficult and changing targe for control measures. Microscopic evidence for sexual reproduction of the rice sheath blight fungus never has been observed in rice fields and it has been thought to be very rare or non-existent in nature. However, recent work described here suggests that the microscopic fungus Rhizoctonia solani may have an active sexual life cycle. Having collected strains of the fungus from various locations in Texas and by using DNA-based methods for measuring the mixing of genetic material, we have been able to determine that sexual reproduction may be an important form of reproduction for the sheath blight fungus. The previously undetected sexual life of Rhizoctonia may make breeding rice plants for managing the disease more difficult in the future and this fact will need to be addressed by plant breeders.

Technical Abstract:
To date, much of the genetics of the basidiomycete Thanatephorus cucumeris (anamorph = Rhizoctonia solani) remains unknown. Here, we present a population genetics approach using co-dominant markers to augment laboratory analyses. Seven single-copy nuclear RFLP markers were used to examine 182 isolates of R. solani AG-1 IA, collected from six commercial rice fields in Texas. Thirty-six multilocus RFLP genotypes were identified. Population subdivision analyses indicated a high degree of gene flow/migration between the six geographic populations. Tests for Hardy-Weinberg equilibrium (HWE) among the 36 multilocus RFLP genotypes revealed that four of the seven loci did not significantly differ from HWE. Subsequent analysis demonstrated that departures from HWE at the three remaining loci were due to an excess of heterozygotes. Data presented here suggest that R. solani AG-1 IA is actively outbreeding (heterothallic). Possible explanations for heterozygote excess, which was observed at all seven RFLP loci, are discussed.